That's a somewhat similar construction to mine. I simply counter-bored the top and through-bolted it to the stretchers, leaving the c'bores exposed and it's been fine since December of 1994. Chances are, you'll never notice they're there. I also lag-bolted through the bottom stretchers into my concrete basement floor, which has been great----my bench doesn't budge, even under the most vigorous planing, etc. A sheet of ply glued/screwed to the back makes it absolutely rigid and eliminates ALL tendency to 'rack' when planing as well, besides keeping tools from falling off the back of the lower shelf.
